Synchronous detection of concurrent servo bursts for fine head position in disk drive

ABSTRACT

Synchronous detection of fine position servo burst information with a data transducer having an electrical width not less than about two-thirds a width of a data track within a partial response maximum likelihood (PRML) data channel is disclosed. The servo burst information is recorded on a storage medium as a pair or series of fractional-track-width sinewave concurrent burst patterns and includes an on-track phase generating a position error signal which varies linearly with head displacement about track centerline and at least one off-track phase generating a position error signal which varies linearly with head displacement about a position related to track boundary. The method includes steps of reading the on-track phase and the off-track phase with a head to provide an analog signal stream, amplifying and low pass filtering the signal stream to normalize gain of the signal stream, synchronously quantizing the signal stream to provide synchronous digital burst samples, multiplying the synchronous digital burst samples during servo sampling intervals by a normalization factor generated by a correlation signal generator to provide normalized samples, integrating normalized samples originating during the on-track phase to provide an on-track position error signal, and integrating normalized samples originating during the off-track phase to provide an off-track position error signal. Circuitry implementing the method including a discrete matched filter is also disclosed.

RELATED PATENT

This is related to commonly assigned U.S. patent application Ser. No.08/174,895 filed on Dec. 23, 1993, entitled: "PRML SAMPLED DATA CHANNELSYNCHRONOUS SERVO DETECTOR", now U.S. Pat. No.5,384,671.

FIELD OF THE INVENTION

The present invention relates to positioning a data transducer headwithin a disk drive among a multiplicity of radial data track locations.More particularly, the present invention relates to a method andapparatus for synchronous detection of concurrently recorded servobursts with a partial response, synchronous sampling data detectionchannel to provide fine head position information in a disk drive.

BACKGROUND OF THE INVENTION

Disk drives, particularly but not necessarily magnetic hard disk drives,record blocks of user data in concentric data tracks defined on majorsurfaces of a rotating rigid magnetic disk. A head structure supportsand positions a data transducer head at each selected data track of anassociated data storage surface in order to carry out reading or writingoperations. In order to seek to, and then follow, each selected datatrack, the head positioning structure must perform head trackingoperations, typically by using a form of servo control loop. Mostprevalently, servo loops derive head position information from servosectors which are interspersed or "embedded" within the data tracks. Aseach servo sector is encountered, user data transfer operations aresuspended, and the head then senses prerecorded patterns of magneticflux transitions which may include a unique servo address mark patternto confirm that the head is reading from the servo sector, a coarsetrack number which is typically Gray coded, and fine positioninformation which serves as a head position vernier within each track.

Within disk drives having a multiplicity of concentric data tracksdefined on a rotating magnetic storage surface, head tracking hasconventionally been carried out by asynchronously detecting Gray codetrack number information recorded within embedded servo sectors in orderto provide coarse head position information to a head position servoloop. The servo control loop employed the coarse head positioninformation, particularly during track seeking operations to controlhead trajectory from a departure track to a destination track. Once thedestination track was reached, further information, known as "fineinformation" was needed to provide fine positional adjustments to thehead structure, to be sure that the head was precisely following acenterline of the destination track.

In prior peak detection recording channels, the fine head positioninformation has been embedded within the servo sectors in generally twodifferent ways. One method employs a dibit, tribit or quadrabit patternin which aligned flux transitions recorded in half-track-width patternsare summed and differenced within the magnetic data head structure. Adiscussion of the synchronous bit fine servo approach is provided inU.S. Pat. No. 4,101,942 to Jacques entitled: "Track Following ServoSystem and Track Following Code". Another approach in whichbetween-track positions are capable of being resolved is given in U.S.Pat. No. 4,032,984 to Kaser et al., entitled: "Transducer PositioningSystem for Providing Both Coarse and Fine Positioning". One drawback ofthese prior approaches was that they were particularly susceptible tonoise interfering with the single or few flux transitions providing thefine position information. Another drawback was that they employedasynchronous peak detection techniques which required that the pulsescarrying the head position information be spaced sufficiently far apartso as to eliminate any possibility of intersymbol interference fromadjacent pulses, and the resultant servo patterns required aconsiderable space within the servo sector format. Finally, even thoughsome off-track position resolving capability existed, as described inthe referenced Kaser et al. patent, the off-track signal was highlytransitional and marginal, and was not capable of providing a robust,reliable position value.

Another prior peak detection servo pattern recorded in embedded servosectors has been the plural burst pattern. In this approach, at leasttwo (A and B) servo bursts are recorded in a radially offset,circumferentially staggered arrangement within each servo sector. Thesepatterns were sequentially read by the data transducer head, andresultant relative amplitudes were sampled, peak detected, integratedand then passed into a servo data channel which asynchronously filteredthe e.g. two, three or four separate and sequentially read sine waveburst fields (herein the "A", "B" and "C" bursts) using analogtechniques. A position error signal (PES) was derived by calculating thedifference in playback amplitudes of a selected pair of the availablebursts. Since each of the selected burst pairs is offset in radialposition from the other burst pair, when the head is on-track, equalrelative burst amplitudes are read from each of two bursts, e.g. the Aand C bursts. For thin film read heads, equal amplitudes are read whenapproximately one half of the head is over each burst.

In one prior example, when the head is not close to the trackcenterline, as during track settling following a seeking operation, thehead makes use of a third or B burst to obtain an accurate measurementof the off-track distance. For example, in one preferred pattern,depicted and described in commonly assigned U.S. Pat. No. 5,170,299 toMoon, entitled: "Edge Servo for Disk Drive Head Positioner", burstamplitudes from bursts B and C will be equal when the head is readingexactly 1/4 track width off track centerline. The disclosure of U.S.Pat. No. 5,170,299 is hereby incorporated by reference.

There are several drawbacks stemming from the prior approach. Onedrawback is that the three servo bursts, A, B and C, and the gapsseparating them require considerable area within each servo sector,thereby increasing the servo overhead and reducing the amount of userdata that can otherwise be recorded on the storage surface.

Another drawback is the requirement for separate analog circuitry forservo as in the case of a partial response, maximum likelihood ("PRML")read channel. While the digital peak detection process set forth incommonly assigned U.S. Pat. No. 5,321,559 to Nguyen et al., entitled:"Detection of Embedded Servo Sector Data with PRML Channel in DiskDrive", improved somewhat upon prior approaches by using the channel A/Dfor conversion of peak values, a need remained for a separate analogpeak detection path, a path control and three burst sample and holdcircuits. These added circuits and complexities have added cost withinthe disk drive system. Also, the prior approach used the conventional A,B, and C burst arrangement described above which is not particularlyefficient or compact in terms of disk storage space. The disclosure ofU.S. Pat. No. 5,321,559 is hereby incorporated by reference. A differentasynchronous servo address mark detection method was described in U.S.Pat. No. 5,255,131 to Coker et al., entitled: "Asynchronous ServoIdentification/Address Mark Detection for PRML Disk Drive System".

Synchronous servo detection methods within synchronous user datadetection channels such as PRML hold the promise of increased burstaccuracy as well as more efficient embedded servo sector patterns. Inaddition, by eliminating separate analog burst sampling and detectioncircuitry including in some cases a separate A/D converter forquantizing servo bursts, added cost can be reduced. Also, by usingsynchronous servo detection, the burst fields can be made more compact,thereby increasing the storage area available for user data.

One known method for performing synchronous servo detection is byemploying the PRML read channel. A slightly modified version of dataread mode can be used to detect servo Gray code and obtain the A, B,and/or C burst information, as explained in the related, commonlyassigned, copending Fisher U.S. patent application Ser. No. 08/174,895filed on Dec. 23, 1993, entitled: "PRML Sampled Data Channel SynchronousServo Detector", now U.S. Pat. No. 5,384,671, the disclosure thereofbeing incorporated herein by reference. In this prior approach, thethree burst servo scheme (or a four burst scheme for MR read heads) wasretained. The servo burst sinewave values read by the head were low passfiltered, then digitized by the read channel A/D converter and finallyaccumulated. Due to the unpredictability of the burst amplitude at anygiven time (burst values are acquired to ascertain track position), PRMLtiming and gain loops were not adapted during measurement of the A, Band C burst relative amplitudes. It was thus assumed in this priorapproach that the timing and gain loops would "coast" over the servoburst regions, or that additional synchronization fields would beinserted between the bursts to enable resynchronization of the timingand gain loops thereby adding to the servo overhead on the disk storagesurface. Thus, this earlier solution was less than optimal inperformance and continued to be wasteful in disk data storage area. Eventhough three distinct bursts were recorded in each servo sector, onlytwo different values were of interest to the servo head positioncircuitry at any given time: the on-track position error signal A-C, andthe off-track position error signal B-C.

Thus, a hitherto unsolved need has remained for a more compact andefficient servo burst pattern from which position error signals may bedetected with synchronous sampling techniques in a more efficient mannerthan before.

SUMMARY OF THE INVENTION WITH OBJECTS

A general object of the present invention is to enable directsynchronous detection of fine head position signals recorded at afrequency otherwise resulting in intersymbol interference in a mannerovercoming limitations, drawbacks and disadvantages of the prior art.

Another object of the present invention is to provide a more efficientand compact servo burst pattern adapted for inclusion within an embeddedservo sector and adapted for synchronous detection with a sampling datadetection channel, such as a PRML channel.

Another object of the present invention is to provide an embedded servoburst pattern which includes timing and gain loop information to enableupdating of timing and gain loops of a sampling data detection channelin a manner overcoming limitations and drawbacks of the prior artapproaches.

Another object of the present invention enabling synchronous servo burstdetection is to reproduce phase coherent embedded servo informationwithout loss of phase lock/gain lock synchronism with adjacentlyrecorded embedded servo data.

Another object of the present invention is to provide a new servo burstpattern for an embedded servo sector which eliminates the need for anyDC erase gaps between successive burst fields.

Another object of the present invention is to provide a shortened andmore efficient servo burst pattern which is particularly suited todetection within a partial response, maximum likelihood synchronousdetection channel.

A further object of the present invention is to provide a method andapparatus for synchronous servo burst detection usable with eitherconventional thin film inductive or magneto-resistive read elementsincluded within a hard disk drive sampling data detection channel.

One more object of the present invention is to employ a discrete, gatedmatched filter within a synchronous servo burst detector for detectinghead fine position from embedded servo burst information.

A further object of the present invention is to provide a synchronousservo detection method which includes a single stream of data dividedinto three distinct phases: an on-track phase, at least one off-trackphase, and a coherent phase. These phases may be written only oncewithin the servo sector, or they may be repeated as desired or requiredby a particular servo loop. A plurality of off-track phases may beprovided for use with narrow-read-width magneto-resistive read elements.

In accordance with one facet of the present invention, a method enablessynchronous detection of fine position servo concurrent burstinformation with a data transducer having an electrical width not lessthan about two-thirds a width of a data track within a PRML datachannel. The servo burst information is recorded as e.g. a 1/4T sinewaveburst pattern within each one of a multiplicity of concentric datatracks of a storage disk, the servo concurrent burst pattern being phasecoherent with one or more other servo data fields recorded in anembedded servo sector, the fine position servo concurrent burstinformation including an on-track phase generating a position errorsignal which is linear about track centerline and an off-track phasegenerating a position error signal which is linear about track boundary.The new method comprises the steps of:

reading the on-track phase and the off-track phase with a head toprovide a fine position analog signal,

amplifying and low pass filtering the signal, to normalize gain,

synchronously quantizing the signal stream to provide synchronousdigital burst samples, and

passing the synchronous digital burst samples through a discrete gatedmatched filter structure including the steps of:

multiplying the synchronous digital burst samples during servo samplingintervals by a normalization factor generated by a correlation signalgenerator to provide normalized samples,

integrating normalized samples originating during the on-track phase toprovide an on-track position error signal, and

integrating normalized samples originating during the off-track phase toprovide an off-track position error signal.

In accordance with another facet of the present invention, a synchronousservo burst detector detects a phase-coherent servo burst pattern for ahead position servo loop for controlling radial position of a datatransducer head relative to a moving media defining a multiplicity ofadjacent data storage tracks within a partial response, maximumlikelihood recording and playback channel. The pattern is recorded onthe media in one-half track widths and includes in a sequence anon-track portion generating a position error signal which is linearabout a centerline of each track, and an off-track portion generating aposition error signal which is linear about a boundary between adjacenttracks. The new detector comprises:

the transducer head for reading the servo burst pattern as an analogelectrical signal stream,

an analog read channel for amplifying the analog electrical signalstream,

a low pass filter for low pass filtering the analog electrical signalstream,

an analog to digital converter for synchronously sampling the filteredanalog electrical signal stream and for putting out synchronous raw datasamples, and

a discrete gated matched filter structure comprising:

a correlation signal generator for generating a sequence of synchronousdigital correlation values,

a multiplier for multiplying each synchronous raw data sample by acorresponding digital correlation value to produce a normalizedquantization sample, and

a digital integrator connected to integrate a sequence of normalizedquantization samples obtained from the on-track portion to provide anon-track position error signal to the head positioner servo loop, andconnected to integrate a sequence of the normalized quantization samplesobtained from the off-track portion to provide an off-track positionerror signal to the head positioner servo loop.

These and other objects, advantages, aspects and features of the presentinvention will be more fully understood and appreciated uponconsideration of the following detailed description of a preferredembodiment, presented in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Drawings:

FIG. 1A represents an enlarged, lineal diagrammatic depiction of amagnetization pattern for one servo sector continuous PES field inaccordance with aspects of the present invention. FIG. 1B represents PR4synchronous sample values derived from the FIG. 1A pattern at knownradial sampling positions of the head. FIG. 1C represents a graph ofcross-track position profiles put out by a FIG. 3 correlator-integrator(discrete gated matched filter) circuit, showing on-track and 50%off-track phase signals as a function of head-to-track position.

FIG. 2 represents a series of graphs of analog electrical waveformsinduced in a data transducer head at selected radial head positionsrelative to the FIG. 1A magnetization pattern.

FIG. 3 is a block diagram of a PRML data channel within a hard diskdrive including a synchronous servo burst detection circuit and acorrelator-integrator (discrete gated matched filter) for detecting theFIG. 1A magnetization pattern in accordance with principles of thepresent invention.

FIG. 4A represents an enlarged, lineal diagram of a magnetizationpattern for a servo sector continuous PES field for use with write-wideread-narrow head transducer assemblies such as those having inductivethin film write elements and magnetoresistive read elements, inaccordance with further aspects of the present invention. FIG. 4Brepresents a graph of PR4 synchronous sample values derived from theFIG. 4A pattern. FIG. 4C represents a graph of cross-track positionprofiles put out by a correlator filter circuit, showing on-track, 33%off-track, and 67% off-track phase signals as a function of head-trackposition.

FIG. 5 is a partial block diagram of a PRML data channel within a harddisk drive including a synchronous servo burst detection circuit and acorrelator-integrator for detecting the three-phase magnetizationpattern illustrated in FIG. 4A.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention improves upon the prior art approaches forrecovering fine or vernier head position information from relativeamplitudes of radially offset, circumferentially sequential servo burstpatterns e.g. of constant frequency. The present improvement may berealized in several ways. Essentially, a burst field area 100 withineach embedded servo sector 17 of a data storage disk 16 is coherentlyrecorded (by conventional coherent-phase servo writing techniques) inone half track width radial bands or track increments, as shown in FIG.1A.

FIG. 1A depicts a new servo burst pattern in accordance with principlesof the present invention from a perspective of recorded one-half trackwidth magnetization patterns, wherein a plus symbol denotes e.g.recorded magnetic domains of a first polarity (e.g. N-S) and wherein aminus symbol denotes recorded magnetic domains of an opposite polarity(e.g. S-N) within a longitudinal recording pattern. The servo addressmark and track number patterns are not shown in the FIG. 1A example,however, the servo address mark field 108 is shown in FIG. 2. In thepresent example, three phases of the fine position servo burst portion100 are preferred and therefore illustrated. These phases include anon-track phase 102, a 50% off-track phase 104 and a coherent phase 106between the on-track phase 102 and the 50% off-track phase whichcomprises a repeating phase-coherent magnetization pattern which doesnot vary from track to track with radial displacement of the headrelative to the disk. The coherent phase 106 is provided to enable thesynchronous sampling gain and timing loops to reset to precise timingphase and gain following the on-track phase 102, for recovery of theinformation in the 50% off-track phase 104.

In the case of the on-track phase 102 the magnetization pattern changesat the centerline of each track. Thus, in the on-track phase 102 themagnetization patterns are of opposite polarity. When the head 26 isfollowing the centerline of a track, it nominally electrically subtendseach half width pattern equally, and sums the opposite-phase patterns,resulting in zero amplitude position error signal (PES) samples (theremay be a slight inequivalence in some magneto-resistive read responsecharacteristics).

In the case of the 50% off-track phase 104, the magnetization patternsare of opposite polarity at the track boundary. If, for example, thehead 26 is settling at a destination track location at the end of atrack seeking operation, or the disk drive has sustained a bump or shockforce which drives the head 26 off-track such that the head is nowfollowing a track boundary between adjacent tracks, rather than a trackcenterline, the head 26 will read a full amplitude from the on-trackphase 102, and will realize servo samples representing a net zeroamplitude output (PES=0) from the 50% off-track phase 104.

In the coherent phase 106, the magnetization patterns do not changealong the disk radius (except as the data transfer rates change at zoneboundaries as explained in the related U.S. Pat. No. 5,384,671, eitherat the centerline, or at the boundary, of each track. So, ideally, thecoherent phase 106 provides a reference signal as may be useful in somePRML channels for updating of e.g. the timing, gain, and/or DC offsetcontrol loops.

The on-track phase 102, off-track phase 104 and coherent phase 106 mayinclude full-cycle 1/4T repetitions of the magnetization pattern shownin FIG. 1A to provide a desired number of sample values, so that eachphase 102, 104 and 106 represents a separate "burst". Each of thesephases represents a "concurrent burst" in that the two half-track-widthsinewave portions of each phase are concurrently read. In the presentexample, each full cycle of the sinewave burst includes two positivesamples and two negative samples or 4T samples. By synchronous samplingis meant that the digital samples are ideally taken at e.g. ±0.707 ofthe positive and negative peaks of each full cycle. For example, theremay be 12 4T magnetization patterns (48 clock cycles or T cycles) forthe on-track phase 102 and the off track phase 104; and there may be 44T magnetization patterns (16T cycles) of the coherent phase 106 (torestore timing phase and gain loop lock). Longer or shorter phases maybe employed, depending upon system tolerances and control loop lockcharacteristics. Also, other sinewave periodicities may be employed,such as 6T, 8T, etc.

Thus, as shown in FIG. 1A, if a data transducer head, e.g. the thin filmread/write head 26 of FIG. 3, is precisely following a track centerline,the magnetization patterns will cancel themselves out when the headpasses over the on-track phase 102. During the coherent phase, thepatterns will add together, irrespective of radial displacement of thehead, and the patterns will also add together during the 50% off-trackphase 104 for so long as the head follows each track centerline.

Fig. 1B illustrates the new servo burst pattern from the perspective ofequalized partial response, class IV (PR4) samples, which are obtainedwhen the head is centered either at track center, in head positions 1, 3and 5, or at track boundaries as in head positions 2, 4, and 6, asillustrated in FIGS. 1A and 1B. In the FIG. 1B example, the "X" valuesdenote boundary samples and do not in themselves yield usable PESinformation.

FIG. 1C illustrates graphically two cross-track PES profiles which areput out by a correlation-integration circuit (discrete sample matchedfilter), for the on-track phase 102 and for the 50% off-track phase 104.The correlation-integration circuit is explained below. Since the e.g.thin film inductive read/write head 26 has in the present example aneffective electrical read-mode width e.g. of about two-thirds of onetrack width, each PES curve graphed in FIG. 1C includes peak amplitudesaturation regions wherein amplitude remains invariant with change inradial head position. In the on-track phase 102, the saturation regionsnominally occur at the track boundaries, and in the 50% off-track phasethe saturation regions nominally occur at the track centerlines.Fortunately, when samples from on-track phase 102 representpeak-saturation, samples are obtained from the most linear region of the50% off-track phase, and conversely so. In this sense the PES outputsfrom the on-track phase 102 and the 50% off-track phase 104 result inPES amplitude values closely analogous to the values realized anddiscussed in the referenced and commonly assigned U.S. Pat. No.5,170,299. Also, by looking at the polarity of samples of the burstsignal being read from the 50% off-track phase 104, it can beascertained from this phase information whether an odd track or an eventrack centerline is being followed (odd and even being related to anabsolute track numbering system before e.g. Gray coding thereof).

FIG. 2 provides yet another perspective from which to consider the FIG.1A servo burst pattern. This perspective is the electrical analog signalwaveform induced in the head 26 during passage over the servo sector100. In FIG. 2, a synchronous servo address mark field 108 is shownimmediately preceding the on-track phase 102, coherent phase 106 and 50%offset phase 104 in the sense of relative motion between the head 26 andan underlying, rotating data storage disk, such as the disk 16 shown inFIG. 3. In several locations which are marked by circles in FIG. 1A andin FIG. 2, the magnetic dipole patterns are arranged in like-poleconfrontation. This like polar confrontation of magnetic domains markedin the magnetic patterns graphed in FIG. 1A results in phasediscontinuities in the recovered analog electrical signal, as shown inthe also-marked portions of the FIG. 2 waveforms.

Given the fact that the head 26 manifests an effective electrical widthof about two-thirds track width, the FIG. 2 scheme employing an on-trackphase and a 50% off-track phase is adequate to enable the head 26accurately to resolve head position for proper performance of the headposition servo loop.

With reference to FIG. 3, an exemplary high performance, high datacapacity, low cost disk drive 10 incorporating a programmable andadaptive PRML write/read channel with synchronous servo burst fielddetection in accordance with the principles of the present inventionincludes e.g. a head and disk assembly ("HDA") 12 and at least oneelectronics circuit board (PCB) 14 connected to the HDA 12. The HDA 12includes a base including a spindle assembly to which one or more disks16 are mounted in a stacked relationship. A spindle motor 18 rotates thespindle and disk stack at a predetermined rotational velocity. A headstack assembly 20 forming a rotary actuator mounted to the basepositions each data head 26 relative to a corresponding data storagesurface. A rotary voice coil motor 22 rotates the head stack 20 toposition the heads 26. The HDA 12 may follow a wide variety ofembodiments and sizes. One example of a suitable HDA is given incommonly assigned U.S. Pat. No. 5,027,241. Another suitable HDA isdescribed in commonly assigned U.S. Pat. No. 4,669,004. Yet anothersuitable HDA is described in commonly assigned U.S. Pat. No. 5,084,791.Yet another HDA arrangement is illustrated in commonly assigned,copending U.S. Pat. No. 5,255,136. The disclosures of these patents andthis application are incorporated herein by reference thereto.

A head select/read channel preamplifier 28 is preferably included withinthe HDA 12 in close proximity to the thin film heads 26 to reduce noisepickup. As is conventional, the preamplifier 28 is preferably mountedto, and connected by, a thin flexible plastic printed circuit substrate.A portion of the flexible plastic substrate extends exteriorly of theHDA 12 to provide electrical signal connections with the circuitrycarried on the PCB 14.

The electronics PCB 14 physically supports and electrically connects thedisk drive electronic circuitry needed for an intelligent interface diskdrive subsystem, such as the drive 10. The electronics circuitrycontained on the PCB 14 includes encode/write circuitry 30, driveelectronics including a host interface 32 and a disk drive-host busstructure 34 over which commands and data are passed from the host, andstatus and data are passed from the disk. The drive electronics 32includes e.g. a sequencer, an ENDEC/SERDES, ECC circuitry, a precoder, acache buffer controller, a cache buffer memory array, a high levelinterface controller implementing a bus level interface structure, suchas SCSI II target, for communications over a bus 21 with a SCSI II hostinitiator adapter within a host computing machine (not shown). Anembedded, programmed digital microcontroller controls data formattingand data transfer operations of the sequencer, data block transferactivities of the interface, head positioning of the rotary actuatorstructure 20 via a servo loop including the head 26, the preamplifier 28and synchronous data channel (in a manner explained hereinafter), and ahead position servo control loop circuit 24. The microcontroller alsocontrols the spindle motor 18 via a suitable motor control/drivercircuit. These elements are not a part of the present invention and arenot described in any detail herein.

The channel of FIG. 3 may conventionally include the read preamplifier28, a data path 29 leading to a first variable gain amplifier 36 forcontrolling gain of the analog signal stream on the path 29, an analogprogrammable filter/equalizer 40 which functions to shape the spectrumof the analog signal stream desirably to a partial response spectrum, ananalog adder 42 which is used to control DC offset, a flash analog todigital converter 46, a digital adaptive finite impulse response filter48, a Viterbi detector 52, and a postcoder 54. A dual mode gain controlloop 38, and a dual mode timing loop 50 also provide important functionswithin the synchronous data channel. All of these elements togethercomprise a highly efficient, and presently preferred synchronous PRMLdata channel of a type described in the referenced, commonly assigned,copending U.S. patent application Ser. No. 07/937,064 entitled "DiskDrive Using PRML Class IV Sampling Data Detection with Digital AdaptiveEqualization", now U.S. Pat. No. 5,341,249, the disclosure thereof beingincorporated herein by reference. The DC offset control circuit 44receives unconditioned data samples from the output of the flash A/D 46and determines a DC offset correction value preferably in accordancewith the teachings of commonly assigned, copending Ziperovich U.S.patent application Ser. No. 08/276,817 filed on Jul 18, 1994, entitled:"Real-Time DC Offset Control For PRML Sampled Digital Data Channel", nowU.S. Pat. No 5,459,679 the disclosure thereof being incorporated hereinby reference. The digital DC offset correction values are converted toanalog values by an offset DAC (not shown) and used to adjust the DCoffset at the analog adder 42 in the analog data path between thefilter/equalizer 40 and the flash A/D converter 46.

Alternatively, the synchronous data channel may be realized differently.In one different, yet entirely satisfactory approach, a sample and holdcircuit receiving the incoming data stream from the head 26 may befollowed by an analog magnitude FIR filter, followed by an amplitudedigitizer, and then by a synchronous interleave detector and postcoder,etc.

In the synchronous data channel example illustrated in FIG. 3, magneticflux transitions sensed by the selected data transducer head 26 duringplayback mode are preamplified as an analog signal stream by the readpreamplifier circuit 28. These signals will resemble the analogwaveforms graphed in FIG. 2 herein during passage of servo sectorpattern 100 by the head 26 and depending on radial alignment of the head26 with the particular data track being followed. The preamplifiedanalog signal stream is then sent through the path 29 to the analogvariable gain amplifier (VGA) 36. After controlled amplification, theanalog signal stream is then passed through the programmable analogfilter/equalizer stage 40. The analog filter/equalizer 40 is programmedso that it is optimized for the data transfer rate of a selected radialzone of data tracks from within which the transducer head 26 is readingdata (the zones being arranged to optimize data transfer rate with diskradius in conventional fashion). The equalized analog read signal isthen subjected to sampling and quantization within the high speed flashanalog to digital (A/D) converter 46 which, when synchronized to userdata, generates raw digital data samples {x_(k) }. The A/D 46 isprovided with a suitable resolution, which is at least six bits, andmore preferably, eight bits per sample {x_(k) }.

An FIR filter 48 is included for further filtering and conditioning theraw data samples {x_(k) }. The FIR filter 48 employs adaptive filtercoefficients for filtering and conditioning the raw data samples {x_(k)} in accordance e.g. with desired channel response characteristics (suchas PR Class IV (PR4), EPR4 or E² PR4) in order to produce filtered andconditioned samples {y_(k) }. Since the preamble comprises a sinewavepattern, it is not necessary to equalize to a PR4 response.

The filtered and conditioned data samples {y_(k) } leaving the FIRfilter 48 pass over the path 49 to the Viterbi detector 50 which decodesthe data stream, based upon the Viterbi maximum likelihood algorithmemploying a lattice pipeline structure implementing a trellis statedecoder, for example. At this stage, the decoded data put out on a path96 is passed through a postcoder 52 which restores the original binarydata values.

Use of a digital gain control loop and a digital timing control loop hasa number of advantages. First, a gain (and timing) control loopcontrolled via digital samples is less sensitive to variations intemperature, power supply and component tolerances than is a strictlyanalog control loop. Second, loop compensation or bandwidth may beeasily adjusted or varied simply by loading registers, or by switchingbetween banks of registers for "on the fly" changes. This means thatoptimal loop compensation may be used during both acquisition andtracking modes. Finally, the digital loop filter eliminates errorsotherwise due to bias and offset that may exist in an analog-only loopfilter implementation. In the present invention, ideally, the gain andtiming control loops are able to be updated during passage over eachcoherent phase 106, so that there is no loss of gain or timing controlbetween the on-track phase 102 and the 50% off-track phase 104.

Upon detection of each servo SAM in the field 108, a signalresynchronizes a sector interval timer within a servo sector timingcircuit 58. Following detection of the SAM in the field 108, the timingcircuit 58 marks in time the locations of the on-track phase 102, 50%off-track phase 104 and coherent phase 106, and provides timing andcontrol signals to control a servo Gray code decoder circuit 60 and asynchronous servo burst amplitude detector circuit 61 in accordance withthe present invention. Details of the SAM decoder 56, servo sectortiming circuit 58 and servo Gray code detector 60 are given in thereferenced U.S. patent application Ser. No. 08/174,895 referred toabove.

Essentially, the synchronous servo burst detector 61 comprises acorrelator-integrator circuit which manifests in discrete time thecharacteristics of a matched filter which has an impulse response whichis the time inverse of the incoming signal. The detector 61 includes, inaddition to the partial response channel, a multiplier 62 whichmultiplies filtered digital samples received via an output path 49 fromthe FIR filter 48 by values received from a correlation signal generator64. The digital correlator multiplier signal is matched in frequency tothe written burst signal. Referring to FIG. 1B, when a pattern of +2,+2, -2, -2, +2, +2 is being read, the correlation filter values put outby the correlation signal generator 64 are +1, +1, -1, -1, +1, +1, sothat the product put out by the multiplier 62 is an amplitude value inwhich the sign has been selectively complemented. The relative phase ofthe correlator multiplier signal to the readback burst signal may beused to determine off-track direction. The correlation signal generatorcircuit 64 is reset following detection of SAM by a reset control signalpassed from the servo sector timing circuit 58 via a control path 90.

The products put out by the multiplier 62 are selectively accumulated ina PES accumulator circuit 66. With the illustrated arrangement of acorrelator generator 64, multiplier 62, and an accumulator 66, adiscrete time domain matched filter is achieved which produces positionsamples in which output peak signal to noise values are maximizedrelative to a Gaussian noise distribution. In other words, when amultiplicity of samples having had their signs selectively complementedare accumulated by the accumulator 66, the resultant summation providesa very robust head fine position signal in which a noise component hasbeen minimized.

The accumulator circuit 66 includes a summing circuit 68, a latch 70 anda feedback path 72 from an output of the latch 70 to the summingjunction 68. The accumulator circuit 66 is cleared by a CLEAR signal ona path 74 and then enabled by an ENABLE BURST signal on a path 76 fromthe servo sector timing circuit 58. The ENABLE BURST control signal isasserted after several initial data samples of the on-track phase(denoted "X,X" in FIG. 1B) have passed by as to be operative during thebalance of the on-track phase 102 to accumulate an on-track PES andprovide the on-track PES to an on-track PES latch 78. The on-track PESlatch 78 is enabled by an ENABLE ON-TRACK control on a path 80 from theservo sector timing circuit. The latched on-track PES is delivered tothe head position servo control loop circuit 24 via a path 82. Theinitial samples denoted X,X in FIG. 1B are avoided because of potentialphase discontinuities in the sinewave patterns at the boundaries of thephases 102, 104 and 106 which would result in asymmetrical sampling ofthe position samples (sample phase discontinuities). One advantage ofthe present discrete matched filter arrangement is that the correlationsignal generator 64 and the accumulator circuit 66 may be selectivelyturned on during phases 102 and 104, and turned off during the coherentphase 106 without introduction of any unwanted artifacts or aliaseswhich could accompany switching in and out of an analog matched filter,for example.

Responsive to the CLEAR control signal on the path 74 and to the ENABLEBURST control signal on the path 76, the accumulator circuit 66 iscleared and then enabled after several initial samples of the 50%off-track phase (also denoted "X,X" in FIG. 1B) have passed by as to beoperative during the 50% off-track phase 104 to accumulate an off-trackPES and provide the off-track PES to an off-track PES latch 84. Theoff-track PES latch 84 is enabled by an ENABLE OFF-TRACK control on apath 86 from servo sector timing circuit 58. Once enabled, the off-trackPES latch 84 latches the 50% off-track PES value and supplies it to thehead position servo control loop circuit 24 via an output path 88.

FIG. 1C graphs the on-track PES put out on the path 82, and graphs the50% off-track PES put out on the path 88, as a function of radial headdisplacement relative to the concentric data tracks. As shown in FIG.1C, the on-track PES is most linear at each track centerline, whereasthe 50% off-track PES is most linear at each track boundary. The headposition servo control loop circuit selects between the on-track PES andthe 50% off-track PES for fine head position, depending upon which PESis in its linear range.

While in the FIG. 3 embodiment the multiplicand feeding the multiplier62 is taken from the output of the FIR filter 48 via the path 49, itshould be understood that the FIR filter 48 is optional for synchronousservo burst detection. It should be understood by those skilled in theart that as the burst comprises a sinewave pattern, equalization to aparticular partial response spectrum response is not required. Thefiltering and equalization should be selected and applied in order tominimize distortion occurring at the transitions between DC erase zonesand burst zones, for example. If the channel is not effectivelyequalized, when a transition from DC erase to the 1/4T burst pattern isreached, resultant distortion will span multiple raw data samples takenin the transition region. If such distortion is present, the servo burstintervals must be extended, thereby unnecessarily and inefficientlyextending the overhead areas at the expense of user data storage. Themultiplicand could be provided by unconditioned data samples x{k} on thepath 47 at the output of the flash analog to digital converter 46.However, since the FIR filter 18 is present, it makes good sense to useit for further filtering of the data samples.

FIGS. 4A, 4B and 4C illustrate a servo burst pattern which is adapted tohead structures which have electrical widths which are effectively lessthan e.g. about two-thirds of a track width, as is frequently the casewith write-wide/read-narrow thin film inductive write-magnetoresistiveread data transducer heads. When a narrower electrical read width ispresent, the peak amplitude saturation region for each pattern isextended, and the linear region is reduced. In this example, servopatterns may be written in one-third track widths, so that there aree.g. three patterns within the width of each data track, measured fromcenterline to centerline. In this pattern an on-track phase 112 and acoherent phase 114 are followed by a 33% off-track phase 116 and a 67%off-track phase 118. FIG. 1B shows the synchronous data samples. Theon-track phase provides a linear PES signal for 33% of the track widthfrom track centerline. The next 33% of the track width is covered by the33% off-track phase 116, and the final 33% is provided by the 67%off-track phase, with some overlap of the linear ranges of each phase,as shown in FIG. 4C, as determined by the effective electrical readwidth of the data transducer head. In order to implement this particularapproach, the detector circuit 61 of FIG. 3 includes an additionalaccumulator circuit, and the timing circuit 58 provides three sequentialreset and accumulation enable control signals to control operations ofthe three accumulators.

FIG. 5 illustrates a synchronous servo burst detection circuit 161 whichhas been adapted to detect the FIG. 4A pattern. The servo burst matchedfilter detection circuit 161 is particularly useful when a writewide/read narrow head structure is employed in which the effectiveelectrical width of the read element is less than two-thirds of thetrack width or track width. Accordingly, the FIG. 5 embodiment isparticularly useful within disk drives employing inductivewrite/magnetoresistive read head transducer assemblies, for example. Inthe FIG. 5 example circuit elements which are unchanged from the FIG. 3embodiment bear the same reference numerals, and other unchangedelements are omitted from the discussion of FIG. 5. The explanationgiven above for the same elements applies with equal force to the FIG. 5example.

In the modified discrete matched filter detector 161, a multiplier 162multiplies the filtered digital samples taken from the FIG. 4A patternby values received from a correlation signal generator 164, aspreviously explained above for the FIG. 3 example. The correlationsignal generator 164 is reset following detection of SAM by a resetcontrol signal passed from the servo sector timing circuit 158 via acontrol path 190.

The products put out by the multiplier 162 are sequentially accumulatedin a PES accumulator circuit 166. The accumulator circuit 166 is activeduring receipt of the samples from the on-track phase 112 of FIG. 4A andincludes a summing circuit 172, a latch 174 and a feedback path 176which feeds the output of the latch 174 back into the summing circuit172 and which also provides an on-track phase PES to an on-track PESlatch 178. The latch 174 is cleared by a CLEAR control signal suppliedvia a path 180 from the servo sector timing circuit 158. The on-trackPES latch 178 is enabled by an ON-TRACK ENABLE control supplied over apath 184 from the servo sector timing circuit 158. An output path 186supplies the on-track PES signal to the head position servo control loopcircuit 124.

After being cleared, the accumulator circuit 166 then accumulatessamples from the 33% off-track phase 116 of FIG. 4A. A 33% off-track PESsignal is then transferred to a 33% off-track PES latch 188 which isenabled by a 33% OFF-TRACK ENABLE control supplied over a path 190 fromthe servo sector timing circuit 158. The accumulated 33% off-track PESin the latch 188 is then supplied over an output path 192 to the headposition servo control loop circuit 124. Then the accumulator latch 174is disenabled by deassertion of the ENABLE BURST control on the path182, and again cleared by the CLEAR signal asserted over the controlpath 180.

During receipt of the samples from the 67% off-track phase 116 of FIG.4A, the accumulator circuit 166 is again enabled by assertion of theENABLE BURST control over the path 182, and accumulates the samples fromthe 67% off-track phase. A resultant 67% off-track PES is then suppliedto a 67% off-track PES latch 194 which is enabled by a 67% OFF-TRACKENABLE control signal supplied via a path 196 from the servo sectortiming circuit 158. An output path 198 from the 67% off-track PES latch194 provides a 67% off-track phase PES to the head position servocontrol loop circuit 124.

Having thus described an embodiment of the invention, it will now beappreciated that the objects of the invention have been fully achieved,and it will be understood by those skilled in the art that many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the spirit andscope of the invention. The disclosure and the description herein arepurely illustrative and are not intended to be in any sense limiting.

What is claimed is:
 1. A method for synchronously detecting fine headposition information recorded within an embedded servo sector within atrack defined on a moving magnetic storage medium as a first pluralityof longitudinal sinewave phase coherent patterns in phase oppositionalong a first longitudinal boundary and a second plurality oflongitudinal sinewave phase coherent patterns in phase opposition alonga second longitudinal boundary and separated from the first plurality bya phase coherent reference pattern within the track, the first andsecond longitudinal boundaries being located in relation to a centerlineof the track, the synchronous detection being accomplished with amagnetic read head having an effective electrical width less than anominal width of the track and a partial response maximum likelihoodsampling data detection channel, wherein the patterns are recorded at arate manifesting intersymbol interference within the sampling datadetection channel, the method comprising the steps of:establishinginitial timing and gain control conditions within the sampling datadetection channel, reading the first plurality of phase coherentpatterns with the magnetic read head to produce a first analog servosignal stream, reading the phase coherent reference pattern to produce areference analog signal stream, reading the second plurality of phasecoherent patterns with the magnetic read head to produce a second analogservo signal stream, analog filtering the first and second analog servosignal streams with an analog filter within the sampling data detectionchannel to produce a pair of filtered analog signal stream sequences,synchronously quantizing the first analog signal stream sequence toprovide first synchronous digital burst samples along the firstlongitudinal boundary, synchronously quantizing the reference analogsignal stream to provide reference synchronous digital burst samples,synchronously quantizing the second analog signal stream sequence toprovide second synchronous digital burst samples along the secondlongitudinal boundary, filtering the first and second synchronousdigital burst samples within a matched filter to maximize output peaksignal to noise to produce filtered fine head position values relativeto track centerline, providing the filtered fine head position values toa head positioner circuit for positioning the magnetic read headrelative to track centerline, and using the filtered reference samplesto update the timing and gain control conditions within the samplingdata detection channel before the second plurality of phase coherentpatterns are read.
 2. The method set forth in claim 1 wherein the stepof filtering the first and second synchronous digital burst sampleswithin a matched filter comprises the steps of:multiplying the first,reference and second synchronous digital burst samples by anormalization factor generated by a correlation signal generator toprovide normalized samples, integrating the normalized samples within anintegrator to produce the fine head position values, and holding thefine position values in a plurality of latches for use by the headpositioner circuit.
 3. The method set forth in claim 1 wherein theembedded servo sector includes at least a timing synchronization fieldpreceding the phase coherent burst patterns and wherein the step ofestablishing initial timing and gain control conditions within thesampling data detection channel includes the further step of phaselocking a quantization timing loop of the sampling data detectionchannel in synchronism with the synchronization field.
 4. The method setforth in claim 1 wherein magnetic read head has an effective electricalread width at least about two-thirds of a width of the data track, andwherein the longitudinal boundary of the first longitudinal boundary. iscoincident with the centerline of the data track.
 5. The method setforth in claim 1 wherein the magnetic read head has an effectiveelectrical read width less than about two-thirds of a width of the datatrack, wherein the first longitudinal boundary of the is coincident withthe centerline of the data track, wherein the second longitudinalboundary is offset from the centerline of the data track by a firstfractional track width amount and further comprising a third pluralityof longitudinal sinewave phase coherent patterns in phase oppositionalong a third longitudinal boundary and separated from the secondplurality by a second phase coherent reference pattern within the track,and comprising the further steps of:reading the third plurality of phasecoherent patterns with the magnetic read head to produce a third analogservo signal stream, analog filtering the first, second and third analogservo signal streams with the analog filter within the sampling datadetection channel to produce a series of filtered analog signal streamsequences, synchronously quantizing the third analog signal streamsequence to provide third synchronous digital burst samples along thethird longitudinal boundary, filtering the third synchronous digitalburst samples within a matched filter to maximize output peak signal tonoise to produce a filtered third fine head position value relative totrack centerline, and providing the filtered third fine head positionvalue to a head positioner circuit for fine positioning the magneticread head relative to track centerline.
 6. A method for synchronouslydetecting fine position servo burst information with a data transducerhaving an electrical width not less than about two-thirds a width of adata track within a partial response maximum likelihood data channelwherein the servo burst information is recorded as a concurrent burstpattern of two half-track-width phase coherent sinewaves within each oneof a multiplicity of concentric data tracks of a disk, each patternincluding an on-track phase for generating a position error signal whichvaries approximately linearly with head displacement about trackcenterline and an off-track phase generating a position error signalwhich varies approximately linearly with head displacement about trackboundary, comprising the steps of:reading the on-track phase and theoff-track phase with a head to provide an analog signal stream,amplifying and low pass filtering the signal stream, to normalize gainof the signal stream, synchronously quantizing the signal stream toprovide synchronous digital burst samples, multiplying the synchronousdigital burst samples during servo sampling intervals by a normalizationfactor generated by a correlation signal generator to provide normalizedsamples, integrating normalized samples originating during the on-trackphase to provide an on-track position error signal and holding theon-track position error signal, integrating normalized samplesoriginating during the off-track phase to provide an off-track positionerror signal and holding the off-track position error signal and readingand quantizing a coherent phase pattern in the data track following theon-track phase and preceding the off-track phase to produce referenceburst samples and using the reference burst samples for updating atiming control loop within the partial response maximum likelihood datachannel before synchronously quantizing a portion of the signal streamcorresponding to the off-track phase.
 7. The method set forth in claim 6comprising the further step of equalizing the synchronous digital burstsamples before the multiplying step is carried out.
 8. The method setforth in claim 6 wherein the step of amplifying and low pass filteringthe signal stream includes the step of normalizing gain of the signalstream with a gain control loop responsive to the synchronous digitalburst samples.
 9. The method set forth in claim 6 wherein the step ofsynchronously quantizing the signal stream includes the step ofcontrolling an analog to digital converter with the timing control loopresponsive to the synchronous digital burst samples.
 10. The method setforth in claim 6 wherein the servo burst information is recorded asphase coherent half track width patterns bounded by longitudinal signalnull positions located in relation to track centerline.
 11. The methodset forth in claim 6 comprising the further steps of updating gainnormalization of the signal stream with a gain control loop within thepartial response maximum likelihood data channel and responsive to thesynchronous digital burst samples taken from the coherent phase.
 12. Themethod set forth in claim, 6 comprising the further step of providing aservo address mark within a servo sector and preceding in time the servoburst information, detecting the servo address mark and thereuponresetting a servo sector timing circuit within the partial responsemaximum likelihood data channel, and controlling integration timing ofthe integration steps with the servo sector timing circuit.
 13. A methodfor synchronously detecting fine position servo burst information with adata transducer having an electrical width less than about two-thirdswidth of a data track within a partial response maximum likelihood datachannel wherein the servo burst information is recorded as phasecoherent sinewave, one-third track width patterns bounded between trackcenterline within each one of a multiplicity of concentric data tracksof a disk, each pattern including an on-track phase generating aposition error signal which varies approximately linearly with headdisplacement about track centerline, a 33% off-track phase generating aposition error signal which varies approximately linearly with headdisplacement about a location one third track width displaced from trackcenterline, a 67% off-track phase generating a position error signalwhich varies approximately linearly with head displacement about alocation two thirds track width displaced from track centerline, and acoherent reference phase separating the on-track phase from an adjacentone of the 33% off-track phase, the 33% off-track phase and the 67%off-track phase, comprising the steps of:reading the on-track phase, the33% off-track phase, the 67% off-track phase and the coherent referencephase with a head to provide an analog signal stream, amplifying and lowpass filtering the signal stream to normalize gain of the signal stream,synchronously quantizing the signal stream with synchronous quantizationmeans to provide synchronous digital burst samples, multiplying thesynchronous digital burst samples from the on-track phase, 33% off-trackphase and the 67% off-track phase during servo sampling intervals by anormalization factor generated by a correlation signal generator toprovide normalized samples, integrating normalized samples originatingduring the on-track phase to provide an on-track position error signaland holding the on-track position error signal, using the digital burstsamples from the reference phase to update synchronization of thesynchronous quantization means before synchronously quantizing portionsof the signal stream representing the adjacent one of the 33% off-trackphase and the 67% off-track phase, integrating normalized samplesoriginating during the 33% off-track phase to provide a 33% off-trackposition error signal and holding the 33% off-track position errorsignal, and integrating normalized samples originating during the 67%off-track phase to provide a 67% off-track position error Signal andholding the 67% off-track position error signal.
 14. The method setforth in claim 13 wherein the step of reading the on-track phase, the33% off-track phase, the 67% off-track phase, and the reference phase iscarried out with a magnetoresistive read head.
 15. The method set forthin claim 13 comprising the further step of equalizing the synchronousdigital burst samples taken from the on-track phase, 33% off-track phaseand 67% off-track phase before the multiplying step is carried out. 16.A synchronous servo burst detector for detecting a phase-coherentsinewave servo burst pattern for a head position servo loop forcontrolling radial position of a data transducer head relative to amoving media defining a multiplicity of adjacent data storage trackswithin a partial response, maximum likelihood recording and playbackchannel, the pattern being recorded on the media in fractional trackwidths and including in a sequence an on-track portion generating aposition error signal which varies approximately linearly with headdisplacement about a centerline of each track an off-track portiongenerating a position error signal which varies approximately linearlywith head displacement about a location offset from track centerline andrelated to a boundary between adjacent tracks, the off-track portionbeing separated from the on- track portion by a coherent reference phaseportion, the detector comprising:the transducer head for reading theservo burst pattern as an analog electrical signal stream, an analogread channel for amplifying the analog electrical signal stream, a lowpass filter for low pass filtering the analog electrical signal stream,an analog to digital converter for synchronously sampling the filteredanalog electrical signal stream and for putting out synchronous raw datasamples, the analog to digital converter controlled by a timing controlloop responsive to synchronous raw data samples from at least thecoherent reference phase portion for generating quantization timingcontrol values for controlling synchronous quantization of the analogelectrical signal stream during at least reading of the off-trackportion, a matched filter for filtering the synchronous raw data samplesfrom the on-track portion and for filtering the synchronous raw datasamples from the off-track portion, an on-track position error signallatch for holding the on-track position error signal and for supplyingit to the head positioner servo loop, and at least one off-trackposition error signal latch for holding the off-track position errorsignal and for supplying it to the head positioner servo loop.
 17. Thesynchronous servo burst detector set forth in claim 16 wherein thematched filter comprises:a correlation signal generator for generating asequence of synchronous digital correlation values, a multiplier formultiplying each synchronous raw data sample by a corresponding digitalcorrelation value to produce a normalized quantization sample, and adigital integrator connected to integrate a sequence of normalizedquantization samples obtained from the on-track portion to provide anon-track position error signal, and connected to integrate a sequence ofthe normalized quantization samples obtained from the off-track portionto provide at least one off-track position error signal.
 18. Thesynchronous servo burst detector set forth in claim 16 furthercomprising a servo address mark pattern recorded in a servo area of themedia adjacently before the servo burst pattern and a servo address markpattern detector for detecting the servo address mark, and a servotiming circuit including an internal servo area timer which is resetupon detection of the servo address mark pattern for timing incrementsincluding the on-track portion and the off-track portion, and forresetting and enabling the digital integrator and the on-track positionerror signal latch and the off-track position error signal latch, andfor resetting the correlation signal generator.
 19. The synchronousservo burst detector set forth in claim 16 wherein the analog readchannel includes a first variable gain amplifier .and a gain controlloop responsive to synchronous raw data samples for generating gaincontrol values for controlling gain of the first variable gainamplifier.
 20. The synchronous servo burst detector set forth in claim16 wherein the analog read channel includes a second variable gainamplifier and a DC offset control loop responsive to synchronous rawdata samples for generating DC offset control values for controlling DCoffset of the filtered analog electrical signal stream immediatelybefore it is synchronously quantized by the analog to digital converter.21. The synchronous servo burst detector set forth in claim 16 whereinthe moving media comprises a magnetic data storage disk, and themultiplicity of tracks are defined to be concentric with an axis ofrotation of the disk.
 22. The synchronous servo burst detector set forthin claim 21 wherein the phase coherent servo burst pattern is includedwithin each one of a plurality of circumferentially spaced-apart servosectors embedded within the concentric data tracks.
 23. The synchronousservo burst detector set forth in claim 16 further comprising a digitalFIR filter connected between the digital to analog converter and themultiplier, the digital FIR filter for equalizing the synchronous rawdata samples in accordance with a predetermined equalizationcharacteristic.
 24. The synchronous servo burst detector set forth inclaim 16 wherein the phase coherent sinewave servo burst pattern isrecorded on the media in a plurality of fractional track widths andincludes at least three fractional width burst patterns bounded betweentrack centerline within each one of a multiplicity of concentric datatracks of a disk, each pattern of a data track being phase coherent withdata recorded in the data track and including an on-track portiongenerating a position error signal which is linear about trackcenterline, a first fractional off-track portion generating a positionerror signal which is linear about a location a first fractional trackwidth displaced from track centerline, and a second fractional off-trackportion generating a position error signal which is linear about alocation a second fractional track width displaced from trackcenterline, the digital integrator connected to integrate a sequence ofnormalized quantization samples obtained from the on-track portion toprovide the on-track position error signal, connected to integrate asequence of the normalized quantization samples obtained from the firstfractional off-track portion to provide a first fractional portionoff-track position error signal, and connected to integrate a sequenceof the normalized quantization samples obtained from the secondfractional off-track portion to provide a second fractional portionoff-track position error signal, wherein the at least one off-trackposition error signal latch holds the first fractional portion off-trackposition error signal, and further comprising a second fractionalportion off-track position error signal latch for holding the secondfractional portion off-track position error signal and for supplying itto the head positioner servo loop.
 25. The synchronous servo burstdetector set forth in claim 24 wherein the first fractional off-trackportion is recorded at a 33% off-track width, and wherein the secondfractional off-track portion is recorded at a 67% off-track width.